Oxidation of cholesterol

The nuclear changes involved in the conversion of cholesterol to cholic acid (reduction of the 5,6 double bond, epimerization of the hydroxyl group at carbon-3, and insertion of hydroxyl groups at positions 7 and 12) appear to be carried out by microsomal enzyme systems. Side-chain oxidation is effected by mitochondrial enzymes. 

It has been shown (47-49) that suitably fortified preparations of mouse or rat liver mitochondria can oxidize the terminal methyl group of cholesterol to CO2. The acidic products obtained are not identical with common bile acids (50,51). When the substrate is 3a,7a,12a-trihydroxycoprostane, however, the product of mitochondrial oxidation is cholic acid (52-54). Danielsson (55,56) has reviewed much of this work. 

The mitochondrial enzyme system does not exhibit a great deal of specificity, being able to oxidize cholesterol, coprostanol, cholestenone, cholestanone, coprostanone (57), and even ergosterol (58) and desmosterol (59). 

With use of the system defined by Whitehouse et ai (49,60), investigations have been carried out on the effects of cholesteropenic drugs on the oxidation of cholesterol-26- C to C02. While the substrate is not the natural one for mitochondrial oxidation, on the presumption that the natural enzyme system(s) is operating, it was thought that some useful clues to mechanism of action might be obtained. 

The first experiments were carried out with nicotinic acid (61), and it was found that mitochondrial oxidation of cholesterol was enhanced in rats fed this compound. It was later found (62) that addition of nicotinic acid (8 X 10 moles, 10 mg) to the incubation mixture enhanced oxidation of cholesterol. Of a large series of nicotinic acid and nicotinamide homologues tested, only 3-pyridylacetic acid also stimulated oxidation of cholesterol (62,63). This compound has also been shown to be hypocholesterolemic in man (64,65). Pyridinol carbamate, 2,6-bis(hydroxymethyl)pyridine-di-A -methylcarbamate, fed to rats as 1 % of their diet also enhanced in vitro oxidation of cholesterol by rat liver mitochondria (66). 

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Figure C1.5.17.(A) Enzymatic cycle of cholesterol oxidase, which catalyses tire oxidation of cholesterol by molecular oxygen. The enzyme s naturally fluorescent FAD active site is first reduced by a cholesterol substrate,...

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

The oxidation of cholesterol to cholest-4-ene-3-one is carried ont by an oxidase in several bacteria. This activity has been fonnd in Brevibacterium sterolicum and Streptomyces sp. strain SA-COO (Ohta et al. 1991), and the extracellnlar enzyme that has been purified from Pseudomonas sp. strain ST-200 (Donkyn and Aono 1998) has a preference for 3p-hydroxy componnds. 

Extensive studies in vitro from many groups have confirmed that exposure of LDL to a variety of pro-oxidant systems, both cell-free and cell-mediated, results in the formation of lipid hydroperoxides and peroxidation products, fragmentation of apoprotein Bioo, hydrolysis of phospholipids, oxidation of cholesterol and cholesterylesters, formation of oxysterols, preceded by consumption of a-tocopherol and accompanied by consumption of 8-carotene, the minor carotenoids and 7-tocopherol. 

The rare example of synergistic action of a binary mixture of 1-naphthyl-A-phcnylaminc and phenol (1-naphthol, 2-(l,l-dimethylethyl)hydroquinone) on the initiated oxidation of cholesterol esters was evidenced by Vardanyan [34]. The mixture of two antioxidants was proved to terminate more chains than both inhibitors can do separately ( > /[xj). For example, 1-naphtol in a concentration of 5 x 10 5 mol L-1 creates the induction period t=170s, 1 -naphthyl-A-phenylamine in a concentration of 1.0 x 10-4 mol L 1 creates the induction period t = 400s, and together both antioxidants create the induction period r = 770 s (oxidation of ester of pelargonic acid cholesterol at 7= 348 K with AIBN as initiator). Hence, the ratio fs/ZfjXi was found equal to 2.78. The formation of an efficient intermediate inhibitor as a result of interaction of intermediate free radicals formed from phenol and amine was postulated. This inhibitor was proved to be produced by the interaction of oxidation products of phenol and amine. 

Zaborowska, Z., Uchman, W., Bilska, A., Jelen, H., Rudzinska, M., Wqsowicz, E. and Kummerow, F.A. (2001). Effect of storage on oxidation of cholesterol and lipids in liver pate type sausage, Electron. J. Polish Agricu. Universities, Series Food Sci. TechnoL, 4, 2. 

Nelson, T.J., Alkon, D.L. (2005) Oxidation of cholesterol by amyloid precursor protein and P-amyloid peptide. J. Biol. Chem., 280,7377-7387. 

SCHEME 6. In vivo oxidation of cholesterol (R = H) and other Upids (R = fatty acyl group) under the influence of UV radiation, by two different paths... 

Steroid Hormones Are Formed by Side-Chain Cleavage and Oxidation of Cholesterol... 

Cholestenone has been prepared by oxidation of cholesterol dibromide with chromic acid 1 or potassium permanganate2 and debromination, and by dehydrogenation of cholesterol over copper oxide 3 4 or by the method described above.5... 

Mauthner and Suida 2 isolated, as one of three neutral products resulting from the oxidation of cholesterol with aqueous chromic acid in acetic acid solution, the substance later identified as A4-cholesten-3,6-dione. The present procedure is based upon results of a reinvestigation of the oxidation by a low-temperature, non-aqueous procedure.3... 

The Oppenauer oxidation of cholesterol to A4-cholesten-3-one of m.p. 77-79° in 70-93% yield has been reported in these volumes.10 Isomerization of As-cholesten-3-one by a mineral acid or a base has been conducted satisfactorily only on a micro scale 6 the method of isomerization with oxalic acid has been reported.3... 

Inspection of Table 6.1 shows that the classical oxidation of sterols on the alcohol at the 3-position, using acetone as oxidant, works efficiently thanks to the migration of the alkene. Thus, the oxidation of cholesterol with acetone (E0 = 129 mV) must proceed via the thermodynamically disfavoured A5-cholesten-3-one (E0 = 153 mV) that evolves to the very stable A4-cholesten-3-one (E0 = 63 mV). In fact, acetone lacks oxidizing power for the obtention of many ketones as well as for the preparation of virtually all aldehydes. 

The reaction is illustrated by the oxidation of cholesterol to cholest-4-en-3-one (Expt 5.90) the migration of the double bond from the / , y- to the a, fi-position is a commonly occurring side reaction associated with unsaturated steroids of this structural type. 

The experimental data for oxidation of benzyl alcohol,1 aliphatic primary and secondary alcohols,2 and cholesterol3 with cetyltrimethylammonium (CTA) dichromate indicated that the reactions occur in a reverse micelle system produced by the oxidant. Michaelis-Menten-type kinetics were observed with respect to the reductants. The product of the oxidation of cholesterol depends on the solvent. In dichloromethane, the product is 7-dehydrocholesterol, whereas with dichloromethane containing acetic acid the product is 5-cholesten-3-one. A low kinetic isotope effect, k /ku = 2.81, was observed in the oxidation of methanol- this, combined with the rate data and the reverse solvent isotope effect [ (H20)/fc(D20) = 0.76], suggests that these reactions... 

Randolph et al. (1988) Batch/semibatch Oxidation of cholesterol Cholesterol oxidases Geioeocysticum chrysocreas and Streptomyces sp. 

Randolph, T. W. Blanch, H. W. Prasnitz, J. M. Enzyme-Catalyzed Oxidation of Cholesterol in Supercritical Carbon Dioxide. AIChE J. 1988, 34, 1354-1360. 

Figure 7 Oxidation of cholesterol regiochemistry of the oxidation with singlet oxygen and of autoxidation (50).

Cholesterol Oxidation. In our recent studies on interaction effects in cholesterol oxidation, we observed that various amino acids and phospholipids inhibit, while triacylglycerols accelerate, the oxidation of cholesterol at 180°C. Some phospholipids were protective while others acted as prooxidants at 130°C, and some exhibited acceleration at the beginning of heating followed by protection (Kim, S. K. Li, Y. G. Nawar, W. W., University of Massachusetts at Amherst, unpublished data). 

Probucol lowers the concentration of both LDL and HDL in the plasma. It inhibits the oxidation of cholesterol thereby lowering the development of atherosclerosis. 

It has also been reported that riboflavin generates superoxide anion in milk exposed to fluorescent light and has been implicated in the destruction of other milk components, such as vitamin C, by light (Spikes and Livingstone, 1969 Korycha-Dahl and Richardson, 1979). The exposure of butter to light has been reported to result in the oxidation of cholesterol, giving rise... 

Kumar, N., Singhal, O.P. 1992. Effect of processing conditions on the oxidation of cholesterol in ghee. J. Sci. Food Agric. 58, 267-273. 

Osada, K., Kodama, T., Yamada, K, Sugano, M. 1993a. Oxidation of cholesterol by heating. J. Agric. Food Chem. 41, 1198-1202. 

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